1. Field of the Invention
The present invention relates to a thin film transistor and a manufacturing method thereof, and a display device including the thin film transistor and a manufacturing method thereof.
2. Description of the Related Art
In recent years, thin film transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Thin film transistors are widely used for electronic devices such as ICs (integrated circuits) and electro-optical devices. In particular, thin film transistors are urgently developed as switching elements of image display devices typified by liquid crystal display devices, EL (electro luminescence) display devices, and the like. In an active matrix liquid crystal display device, specifically, voltage is applied to a pixel electrode connected to a selected switching element and an opposite electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the opposite electrode is modulated optically. The optical modulation can be recognized as a display pattern by an observer. An active matrix liquid crystal display device here means a liquid crystal display device which employs a method in which a display pattern is formed on a screen by driving pixel electrodes arranged in matrix using switching elements. An active matrix EL display device here means an EL display device which employs a method in which a display pattern is formed on a screen by driving pixels arranged in matrix using switching elements.
The application range of the active matrix display devices is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, it is demanded that the active matrix display device has high reliability and that a production method of the active matrix display device offers high yield and reduces production cost. As a method for increasing productivity and reducing production cost, simplification of the process can be given.
In an active matrix display device, thin film transistors are mainly used as switching elements. In manufacturing thin film transistors, reduction in the number of photomasks used in photolithography is important for simplification of the process. If one photomask is added, the following steps are further needed: resist application, prebaking, light exposure, development, postbaking, and the like, and moreover other steps before and after the aforementioned steps, such as film formation and etching and further resist removal, cleaning, drying, and the like. The number of steps is significantly increased only by adding one photomask in the manufacturing process. Therefore, many techniques for reducing the number of photomasks in a manufacturing process have been developed.
Thin film transistors are broadly classified into top gate thin film transistors in each of which a channel formation region is provided below a gate electrode and bottom gate thin film transistors in each of which a channel formation region is provided above a gate electrode. It is known that the number of photomasks used in a manufacturing process of a bottom gate thin film transistor is smaller than that in a manufacturing process of a top gate thin film transistor. A bottom gate thin film transistor is generally manufactured using three photomasks.
Many conventional techniques for reducing the number of photomasks use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. Using such complicated techniques may cause various problems, which leads to reduction in yield. Moreover, there has often been no option but to sacrifice electric characteristics of thin film transistors.
As typical means for reducing the number of photomasks in a manufacturing process of a thin film transistor, a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known. As a technique for reducing the number of steps for manufacturing a thin film transistor by using a multi-tone mask, Patent Document 1 is, for example, disclosed.
As a multi-tone mask which is currently in practical use, a three-tone mask is generally used; however, a four-tone mask has also been developed (for example, see Patent Document 2). PS [Citation List]
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. 2003-179069.    [Patent Document 2] Japanese Published Patent Application No. 2007-249198.